The present invention relates generally to vehicular transmissions. More particularly, the present invention relates to electrically variable transmissions with selective operation that can be tailored to the specific use, or uses, to which the vehicle in which the transmission is incorporated is intended to be used. Specifically, the present invention relates to a transmission having only two differential gearing components, typically planetary gear subsets, and two clutches in the nature of torque transfer devices and yet may be selectively operated in an input split mode, a compound split mode, a neutral mode or a reverse mode.
Air quality, the possibility of climate change, and dependency on fossil fuels are great public concerns today. Vehicles now produce roughly a third of air pollutants and consume roughly a third of fossil fuels. A novel transmission system, for use primarily in vehicles, which can help to reduce emissions and fuel consumption may be of great benefit to the public.
Internal combustion engines, particularly those of the reciprocating piston type, currently propel most vehicles. These engines are soundly criticized by public figures and groups as the most flagrant source of air pollution, energy consumption, and climate change. However, internal combustion engines are relatively efficient, compact, lightweight, and inexpensive mechanisms by which to convert fossil fuel into useful mechanical power, so that with the exception of their negative environmental impact they are very suitable for vehicle propulsion. One of the primary reasons that internal combustion engines are a major source of pollution and fuel consumption is their success and widespread use.
The flexibility with which reciprocating piston internal combustion engines operate is another reason for their pollution and energy consumption. Typically, a vehicle is propelled by such an engine, which is started from a cold state by a small electric motor and relatively small electric storage batteries, then quickly placed under the load of effecting propulsion and operating accessory equipment. Such an engine is also operated through a wide range of speeds and a wide range of loads and typically at a fifth of its maximum power. These wide ranges of engine operation require that clean, efficient combustion be maintained through extremes in operating conditionsxe2x80x94an elusive goal.
A vehicle transmission can deliver mechanical power from an engine to the remainder of a drive system, typically fixed gearing, axles, and wheels. A transmission allows some freedom in engine operation, usually through alternate selection of five or six different drive ratios, a neutral selection that allows the engine to operate accessories with the vehicle stationary, and clutches or torque converters that allow smooth transitions between driving ratios to start the vehicle from rest and accelerate to the desired highway speed with the engine turning. Transmission gear selection typically allows power from the engine to be delivered to the rest of the drive system with a ratio of torque multiplication and speed reduction, with a ratio of torque reduction and speed multiplication known as overdrive, or with a reverse ratio.
An electric generator can transform mechanical power from the engine into electrical power, and an electric motor can transform that electric power back into mechanical power at different torques and speeds for the remainder of the vehicle drive system. This arrangement allows a continuous variation in the ratio of torque and speed between engine and the remainder of the drive system, within the limits of the electric machinery. An electric storage battery used as a source of power for propulsion may be added to this arrangement, forming a series hybrid electric drive system.
The series hybrid system allows the engine to operate relatively independently of the torque, speed, and power to propel a vehicle, so as to be controlled for improved emissions and efficiency. This system also allows the electric machine attached to the engine to function as a motor to start the engine and allows the electric machine attached to the remainder of the drive train to act as a generator, recovering energy into the battery by regenerative braking. A series electric drive suffers from the weight of the electric machinery necessary to transform all engine power from mechanical to electrical and from electrical to mechanical, and from the useful power lost in this double conversion.
A power split transmission can use what is commonly understood to be a xe2x80x9cdifferential gearingxe2x80x9d to achieve a continuously variable torque and speed ratio between input and output without sending all power through the variable elements. An electrically variable transmission can use differential gearing to send a fraction of its transmitted power through a pair of electric motor/generators and the remainder of its power through another, parallel path that is all mechanical and direct, of fixed ratio, or alternatively selectable. One form of differential gearing, as is well known to those skilled in this art, may constitute a planetary gear subset. In fact, planetary gearing is usually the preferred embodiment employed in differentially geared inventions, with the advantage of compactness and different torque and speed ratios among all members of the planetary gearing subset. However, it is possible to construct this invention without planetary gears, as by using bevel differential gears or other differential gears.
For example, a set of bevel differential gears found in a typical automobile axle consists of three or four bevel pinions on a carrier and a meshing bevel gear for each axle. To replace the first subset of planetary gearing in the first embodiment of the invention, the carrier of a first set of bevel differential gears would be connected to the input, one bevel gear that would normally be connected to an axle would instead be connected to the first motor, and the other such bevel gear would be connected to the central shaft. Bevel differential gears could likewise replace the second subset of planetary gearing, and so the invention could be embodied without any planetary gears.
A hybrid electrically variable transmission system for a vehicle also includes an electric storage battery, which allows the mechanical output power to vary from the mechanical input power, engine starting with the transmission system and regenerative vehicle braking.
An electrically variable transmission in a vehicle can simply transmit mechanical power. To do so, the electric power produced by one motor/generator balances the electrical losses and the electric power consumed by the other motor/generator. A hybrid electrically variable transmission system in a vehicle includes an electrical storage battery, so the electric power generated by one motor/generator can be greater than or less than the electric power consumed by the other. Electric power from the battery can sometimes allow both motor/generators to act as motors, especially to assist the engine with vehicle acceleration. Both motors can sometimes act as generators to recharge the battery, especially in regenerative vehicle braking.
One of the most successful substitutes for the series hybrid transmission is the variable, two-mode, input-split, parallel, hybrid electric transmission. Such a transmission utilizes an input means to receive power from the vehicle engine and a power output means to deliver power to drive the vehicle. First and second motor/generators are connected to energy storage devices, such as batteries, so that the energy storage devices can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage devices and the motor/generators as well as between the first and second motor/generators.
Operation in a first or second mode may be selectively achieved by using clutches in the nature of torque transfer devices. In one mode the output speed of the transmission is proportional to the speed of one motor/generator, and in the second mode the output speed of the transmission is generally proportional to the speed of the other motor/generator.
In some embodiments of the variable, two-mode, input-split, parallel, hybrid electric transmission a second planetary gear set is employed. In addition, some embodiments may utilize three torque transfer devicesxe2x80x94two to select the operational mode desired of the transmission and the third selectively to disconnect the transmission from the engine. In other embodiments, all three torque transfers may be utilized to select the desired operational mode of the transmission.
As those skilled in the art will appreciate, a transmission system using a power split arrangement will receive power from two sources. Utilization of one or more planetary gear sets permits two or more gear trains, or modes, by which to deliver power from the input member of the transmission to the output member thereof.
Accordingly, there is a need in the art for a large horsepower transmission system which provides maximum power with little additional power provided by the electric storage device. It is also desirable to enhance overall efficiency at high output speeds. These objectives can be achieved by a two-mode, compound-split, electromechanical transmission that provides the desired high efficiency sought for continuous, constant-speed operation as well as high-average power applications.
It is, therefore, one primary aspect of the present invention to provide a new and novel electrically variable transmission that requires only two torque transfer devices to select either an input split mode of operation or a compound split mode of operation.
It is another aspect of the present invention provide a new and novel electrically variable transmission, as above, that provides the functionality of prior known electrically variable transmissions incorporating three planetary gear subsets and with considerably less complexity.
It is a further aspect of the present invention to provide a new and novel electrically variable transmission, as above, that requires only two planetary gear subsets to provide advantageous torque multiplication from an engine input and two motor/generators that was available only in previously known electrically variable transmissions employing at least three planetary gear subsets.
It is a still further aspect of the present invention to provide a new and novel electrically variable transmission, as above, that provides a neutral mode as well as a reverse mode of operation without the need to employ a third torque transfer device.
It is yet another aspect of the present invention to provide a new and novel electrically variable transmission, as above, that is significantly less complex than prior known electrically variable transmissions.
It is an even further aspect of the present invention to provide a new and novel electrically variable transmission, as above, that can be manufactured at a significant cost reduction relative to prior known electrically variable transmissions.
These and other aspects of the invention, as well as the advantages thereof over existing and prior art forms, which will be apparent in view of the following detailed specification, are accomplished by means hereinafter described and claimed.
By way of a general introductory description, an electrically variable transmission embodying the concepts of the present invention has an input member to receive power from an engine and an output member to deliver power to the drive members that propel the vehicle. There are first and second motor/generators as well as first and second planetary gear subsets. Each planetary gear subset has an inner gear member and an outer gear member that meshingly engage a plurality of planet gear members rotatably mounted on a carrier. The input member is operatively connected to one member of the first planetary gear subset, and the output member is operatively connected to one member of the second planetary gear subset. One motor/generator is operatively connected to another member in the first planetary gear subset as well as being selectively connected to a member of the second planetary gear subset. The second motor/generator is continuously connected to the remaining member of the first and second planetary gear subsets.
In both preferred embodiments, a first torque transfer device selectively grounds one member of the second planetary gear subset, and a second torque transfer device selectively connects this same member of the second planetary gear subset to the inner gear member of the first planetary gear subset as well as to the rotor of one motor/generator.